Dawn Makrinikolas has been a letter carrier in North Attleborough for 10 years. She loves her job - sorting mail in the morning, going on the road with her deliveries, chatting with customers she serves.

But Makrinikolas, 47, is a part-time employee who has been losing hours and earnings as mail volume drops. Every year, when the Postal Service surveys business on its rural routes, she worries her paycheck could shrink further - especially this year, as the service considers drastic cutbacks.

“I can’t sleep for weeks until the count’s over,’’ said Makrinikolas, whose hours were just reduced to 30 a week. “The big picture’s really scary right now.’’

Faced with a $5.1 billion loss last year, the Postal Service is considering slowing first-class mail delivery, closing 3,700 of more than 32,000 post offices, and shuttering 223 of more than 400 mail-processing plants (including 43 post offices and five plants in Massachusetts).

The closings could be finalized as soon as May 15.

If no changes are made, the Postal Service, which generates its own funds and uses no taxpayer dollars, estimates its net loss will reach $21 billion by 2016. Members of Congress have introduced bills to address these money woes, and Congress has final say on some actions, including a proposal to end deliveries on Saturdays.

The changes could have a profound effect on the Postal Service’s 546,000 employees.

“We think that eliminating Saturday delivery would be the beginning of a death spiral for the Postal Service,’’ said John Casciano, business agent for New England for the National Association of Letter Carriers union. “Our competitors would move right in and it would result in tens of thousands of postal employees standing in the unemployment line.’’

About 120,000 positions could be eliminated through attrition and early retirement. Postal Service workers worry the cutbacks will cripple an occupation that not long ago was considered a secure, middle-class career for people with a high school education.

Dennis Avery is a 60-year-old bulk mail clerk at the Waltham processing plant, which is scheduled to close. The 32-year veteran is not worried about being laid off, although he could be transferred. But he worries about the future of the Postal Service because in recent years most of the hiring has been of supplemental workers who don’t get the full pay and benefits of career employees.

“It’s going to be more like a McDonald’s type job,’’ Avery said.

As more people communicate and pay bills online, mail volume has dropped 25 percent since hitting a record 213 billion pieces in 2006.

And while the amount of mail continues to shrink, the number of addresses to which it has to be delivered rises by about 1 million every year as new homes are built and businesses open.

Adding to the financial squeeze is a 2006 congressional mandate that the Postal Service pay an average of $5.5 billion a year into its retiree health care fund from 2007 to 2016.

US Representative Stephen Lynch is cosponsoring a bill that would lower the payments. But, he said, some cuts are inevitable because the post office is at a “breaking point,’’ with emerging technology eroding mail volume even more.

A virtual service called Earth Class Mail, for example, e-mails people photos of their mail and allows them to choose if they want it delivered, scanned, or recycled.

“We’re looking at some changes around the world that don’t bode well for the current design of the Postal Service,’’ Lynch said. “That’s a very good thing environmentally, but it’s a terrible thing for the National Association of Letter Carriers.’’

The Postal Service has been shrinking for years, with less than half the number of post offices today than a century ago and about 250,000 fewer employees than 15 years ago. But this is the first time so many cuts have been proposed at once.

Part of the trouble started when the Postal Service failed to respond to competition from delivery companies such as United Parcel Service of America Inc. and FedEx Corp. said Harlan Platt, a finance professor at Northeastern University. Then the Internet came along, dealing what could potentially be a fatal blow.

“When any other business has shrinkage of 25 percent in five or six years, and can probably forecast another 25 percent decline in the next decade, if not more, it obviously has too much of everything,’’ he said. “The way you deal with it is by downsizing. In the long run, the business goes away.’’

The Postal Service says it might never be as big as it once was, but it will carry on.

“We’re not going to be a 790,000-employee agency again, but we do believe we’ll be delivering to every address in America,’’ said Postal Service spokesman Dennis Tarmey. “We will be here in one form or another.’’

But service cuts will drive too many customers away, workers say. With fewer people on staff since earlier cutbacks, clerks already have to deal with longer lines and more frazzled customers. The proposed closings are causing even more anxiety.

“You could go from [being] a window clerk working normal business hours to working in the plant on a midnight shift,’’ said Scott Hoffman, 49, a clerk at the Chestnut Hill Post Office. “We feel like we’re in a liquidation sale. The pieces are being closed down and sold out from under us.’’

Letter carriers are particularly worried about the proposed end of Saturday service. Not only do people rely on timely delivery of the mail for medications, they say, but carriers serve as a sort of guardians of the neighborhood, noticing when someone doesn’t pick up mail and taking calls from people who want them to check on elderly parents.

“Sometimes during the day we’re the only ones they see,’’ said Mary Collier, 43, a Charlestown native who has delivered mail there for 18 years.

“People trust us and they have faith in us. We’re like an American icon. What is going to happen if we start to deteriorate?’’

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